The present invention is related to gas turbine engines, and in particular to variable stator vanes and variable stator vane actuation mechanisms.
Gas turbine engines operate by combusting fuel in compressed air to create heated gases with increased pressure and density. The heated gases are used to rotate turbines within the engine that are used to produce thrust or generate electricity. For example, in a propulsion engine, the heated gases are ultimately forced through an exhaust nozzle at a velocity higher than which inlet air is received into the engine to produce thrust for driving an aircraft. The heated gases are also used to rotate turbines within the engine that are used to drive a compressor that generates compressed air necessary to sustain the combustion process.
The compressor and turbine sections of a gas turbine engine typically comprise a series of rotor blade and stator vane stages, with the rotating blades pushing air past the stationary vanes. In general, stators redirect the trajectory of the air coming off the rotors for flow into the next stage. In the compressor, stators convert kinetic energy of moving air into pressure, while, in the turbine, stators accelerate pressurized air to extract kinetic energy. Gas turbine efficiency is, therefore, closely linked to the ability of a gas turbine engine to efficiently direct airflow within the compressor and turbine sections of the engine. Airflow through the compressor and turbine sections differs at various operating conditions of the engine, with more airflow being required at higher output levels. Variable stator vanes have been used to advantageously control the incidence of airflow onto rotor blades of subsequent compressor and turbine stages under different operating conditions.
Variable stator vanes are typically radially arranged between stationary outer and inner diameter shrouds, which permit the vanes to rotate about trunnion posts at their innermost and outermost ends to vary the pitch of the vane. Typically, the outermost trunnion posts include crank arms that are connected to a unison ring, which is rotated by an actuator to rotate the vanes in unison. The outermost trunnions extend through the outer shroud, typically an engine case, such that the unison ring is positioned outside the engine case, while the vane airfoils are within the engine case, in the stream of the heated gases flowing through the engine. The engine case comprises a rigid structural component necessary for containing the high operational pressures of the engine, while the unison ring only requires enough strength to transmit torque to the crank arms. As such, the unison ring has a tendency to deform when acted upon by the actuator as the unison ring is suspended over the engine case by the crank arms. Typically, bumpers are positioned between the unison ring and the engine case to increase the rigidity of the unison ring. The bumpers link the unison ring to the engine case such that the engine case lends its stiffness to the unison ring, thus retaining the centricity of the unison ring. However, because the unison ring is disposed outside of the engine case and the flow of the heated gases, the engine casing is subject to much higher temperatures than the unison ring, especially when used with variable turbine vanes. As such, the engine case undergoes greater thermal expansion than the unison ring, resulting in a greater increase in the circumference of the engine case. Thus, there is a tendency for the engine case to grow into the unison ring, causing binding with the bumpers that interferes with precise actuation of the variable vanes. There is, therefore, a need for a variable vane actuation mechanism suitable for use in high temperature differential environments such as turbines.